January 18-24 2021
What are the best possible conditions for quarantine?
Measures taken to fight the virus are many and varied. In addition to mask-wearing and social-distancing, several countries have put quarantine measures in place. In France the length of quarantine has been changed from 14 to 7 days because of socio-economic repercussions that the longer period engenders, so the study of the impact of quarantine on PQT (Post-Quarantine Transmission), in other words, the probability of infecting others after the designated period, takes on great importance.
Based on mathematical modelling, the expected PQT was calculated for different quarantine periods, with or without PCR testing. As shown in the diagram below, the probability of the virus being transmitted is reduced if quarantine times are longer. But the impact of quarantine may be enhanced if used in combination with PCR testing: ensuring that a person is not infected by the virus at the end of quarantine is clearly the best solution.
Then the impact of the date of the test in relation to the duration of the quarantine was studied The largest reduction in PQT was observed in the following 3 situations:
- if a test is carried out after a period of quarantine of 7 days maximum.
- if a test is carried out on the fifth day of periods of quarantine of between 8 and 13 days.
- if a test is carried out on the sixth day of periods of quarantine of 14 days or longer.
The probability of virus transmission in different scenarios was also studied: in the case of a person having been in contact with other infected persons, or someone leaving on or coming back from a trip. In both cases, carrying out a test after quarantine was found to be the most practical means of reducing PQT. Of course, management of a person with possible exposure to COVID 19 who is still in the early phase of infection requires longer quarantine time in comparison to a person who has already developed symptoms. Carrying out a test 6 days after the beginning of quarantine is useful, though less effective than a test at the end of quarantine, and better than 14 days of quarantine without a test.
These different models were able to be applied in real situations, notably in the case of the work-force of an off-shore petroleum company. These employees generally work a cycle of 26 work days followed by 16 rest days. At the beginning of the pandemic, when the number of COVID-19 cases was still low, the company imposed a 3-day quarantine on all its staff, in combination with a test at the outset. These measures evolved into 5 or 7 day quarantines, depending on regions, with a test at the outset and another 4 days later.
The results obtained show that increasing quarantine to 7 days with a test at the end can reduce PQT by 98%, and by 93% for a 5 day quarantine. So in order to reduce PQT in the case of quarantine lasting a maximum of 7 days, the best strategy is to test at the end of this period. For longer quarantines, testing on the fifth or sixth day is also effective. However, testing at the outset of quarantine, which is the most common method, is less effective, especially if the date of infection is unknown.